The present process is a dewaxing process for producing very high viscosity index, low pour point lubricating oil base stocks from a mineral oil feed. When preparing a lubricating oil base stock from a mineral oil, viscosity index is generally increased to a target value during an upgrading step using hydrocracking, solvent refining, etc. Pour point is generally reduced to a target value during a dewaxing step, using catalytic or solvent dewaxing. In conventional processes, the viscosity index generally decreases during dewaxing, since conventional dewaxing processes remove high viscosity index wax from the lubricating oil base stock. Improvements in automotive engine design is puffing ever increasing pressure on the quality of motor oils. Demand for low volatility oils having superior low temperature properties is increasing, and refiners are constantly looking for new processes to aid them in meeting current demands.
High quality lubricants should be, and generally are, paraffinic in nature, since paraffins have a high viscosity index. However, normal paraffins, in particular, are waxy in character, and contribute to a high pour point in the oil. Conventional processes for removing these normal paraffins reduce yield of the lubricant, and have a tendency to reduce the viscosity index of the dewaxed oil. The viscosity index may be increased in the lubricating oil base stock by addition of viscosity index improvers. However, viscosity index improvers are expensive, and tend to fragment at conditions of high temperature and high shear, both of which are commonly found in modern automotive engines.
Synthetic lubricants may be used when very low pour point and very high viscosity index lubricants are desired. But the starting materials used to make the synthetic lubricants, and the processes used in manufacturing these lubricants, are very expensive. The need remains for a lubricating oil base stock, having synthetic-like properties but prepared from a mineral oil feed using methods which are similar to those presently employed in refinery processes.
A major breakthrough came with the discovery of new dewaxing catalysts which were found to isomerize rather than crack the wax molecules. Isomerization alters the molecular structure of wax molecules, and generally decreases the pour point of the molecule without significantly changing its boiling point. In contrast to solvent dewaxing and to wax cracking, isomerized molecules are retained in the lubricating oil base stock, increasing yield of lubricating oil base stock without reducing viscosity index significantly. A particularly important group of isomerization catalysts include the silicoaluminophosphate molecular sieves (SAPO). The preparation of silicoaluminophosphate molecular sieves, including SAPO-11, SAPO-31 and SAPO-41, are taught, for example, in U.S. Pat. No. 4,440,871. Dewaxing processes using such molecular sieves are taught in U.S. Pat. No. 4,859,311; U.S. Pat. No. 4,867,862; U.S. Pat. No. 4,921,594; U.S. Pat. No. 5,082,986; U.S. Pat. No. 5,135,638; U.S. Pat. No. 5,149,421; U.S. Pat. No. 5,246,566; U.S. Pat. No. 5,413,695; and U.S. Pat. No. 4,960,504.
SAPO molecular sieves belong to an important class of non-zeolitic 3molecular sieve dewaxing catalysts which are useful as isomerization catalysts for converting wax and wax-like components. Non-zeolitic molecular sieves are microporous compositions that are formed from AlO2 and PO2 tetrahedra which form 3-dimensional crystalline structures, and are described broadly for this use in U.S. Pat. No. 4,906,351 and U.S. Pat. No. 4,880,760.
These catalysts with isomerization and hydroisomerization activity have been found to provide a method for preparing very high viscosity index lubricating oil base stocks from waxy feedstocks in a single reaction step. Producing a C20+ lube oil from olefins, including normal alpha olefins, using an intermediate pore size molecular sieve and at least one Group VIII metal, is taught in U.S. Pat. No. 5,082,986. In U.S. Pat. No. 5,135,638, a waxy feed containing greater than about 50% wax is isomerized over a catalyst comprising a molecular sieve having 1-D pores having a minor axis between about 4.2 Å]and about 4.8 Å and a major axis between about 5.4 Å and about 7.0 Å and at least one Group VIII metal at a pressure of from about 15 psig (103 kPa) to about 2000 psig (13.8 MPa). SAPO-11, SAPO-31, SAPO-41, ZSM-22, ZSM-23 and ZSM-35 are included in U.S. Pat. No. 5,135,638 as intermediate pore size materials which possess the indicated pore geometry. In U.S. Pat. No. 5,282,958, a feed including straight chain and slightly branched chain paraffins having 10 or more carbon atoms is isomerized with an intermediate pore size molecular sieve having a defined pore geometry, crystallite size, acidity and isomerization selectivity. Feeds which may be processed by the method of U.S. Pat. No. 5,282,958 include waxy feeds, which contain greater than about 50% wax. Such feeds are also taught as often containing greater then 70% paraffinic carbon. U.S. Pat. No. 5,376,260 is directed to pour point reduction of a heavy oil which contains naphthenic wax, using SSZ-32. Heavy oils comprising up to 100% wax are taught.
Large pore zeolites represent another class of catalysts which have been taught for wax isomerization. EP 464546 teaches producing a high viscosity index lubricant from a petroleum wax feed having a paraffin content of at least 40 weight percent The catalyst is a low acidity zeolite isomerization catalyst having an alpha value of not more than 20. Zeolite beta which contains boron as a framework component of the zeolite is taught as being preferred. The catalyst in WO 96/26993 is a low acidity large pore zeolite isomerization catalyst having a ratio of SiO2/Al2O3, as synthesized, of at least 200:1. WO 96/13563 teaches an isomerization process for producing a high viscosity index lubricant using a low acidity large pore molecular sieve having a crystal size of less than 0.1 micron, an alpha value of not more than 30 and containing a noble metal hydrogenation component. EP 225053 teaches isomerization dewaxing using a large pore, high silica zeolite dewaxing catalyst, followed by a subsequent dewaxing step which selectively removes the more waxy n-paraffin components. The selective dewaxing step may be either a solvent or a catalyst dewaxing, preferably using highly shape selective zeolite such as ZSM-22 or ZSM-23.
While the intermediate pore size molecular sieves have been shown to be effective for producing high viscosity index lubricating oil base stocks, the need remains for even higher viscosity index products which have been dewaxed to a low pour point.